i18n: complete audit — 55 new keys, all hardcoded strings converted to t()

- Added 4 missing keys: button.cancel, toast.dir_renamed, toast.file_deleted, toast.file_renamed
- Replaced 41 hardcoded showToast() strings with t() calls across 13 JS files
- Replaced 10 hardcoded textContent/title/innerHTML strings with t() calls
- Fixed theme labels: Sombre/Clair -> t('theme.dark')/t('theme.light')
- Fixed vault selector: Tous les vaults -> t('help.all_vaults')
- Both locale files: 983 -> 1032 keys, perfectly matched
- Bidirectional EN<->FR switching verified in browser

frontend/js/dragdrop.js

@@ -235,7 +235,7 @@ async function onDrop(e) {
 
   // Don't move if source and target are the same vault
   if (vault !== targetVault) {
-    showToast('Cross-vault moves are not supported', 'error');
+    showToast(t('dragdrop.cross_vault_unsupported'), 'error');
     onDragEnd();
     return;
   }
@@ -262,7 +262,7 @@ async function onDrop(e) {
       // Refresh the tree preserving expanded state
       await refreshSidebarTreePreservingState();
     } else {
-      showToast('Move failed', 'error');
+      showToast(t('dragdrop.move_failed'), 'error');
     }
   } catch (err) {
     console.error('Drag & drop move failed:', err);